1. Field of the Invention
The present invention relates to high-frequency semiconductor devices used in the range of high-frequency waves such as microwave and millimetric wave, and particularly to a high-frequency semiconductor device having a superior high-frequency characteristic.
2. Description of the Background Art
As the processing speed of information processors improves and the resolution of image processing devices is enhanced, high-speed and large-capacity personal communication in the high-frequency range such as the millimetric wave band of 30 G-300 GHz, or the centimetric wave band and submillimetric wave band of frequencies higher or lower than that is attracting attention recently. In such communication, not only making use of the high-frequency characteristic but development of a high-frequency package with small size, low cost, and short development period is required.
Generally, the high-frequency package often requires sealing in terms of three aspects, that is, electromagnetic wave, hermetic concern, and mechanical concern. In terms of the mechanical aspect, the reason for sealing is similar to that of the ordinary semiconductor package. In terms of the hermetic aspect, the reason for sealing is that variation of moisture and temperature often influences the high-frequency characteristic of the high-frequency semiconductor chip.
With regard to the sealing in terms of electromagnetic wave, in a high-frequency package for frequencies in the millimetric wave band and the band of frequencies higher and lower than that, the factor which is negligible in the high-frequency package of the mobile telephone or PHS (Personal Handy-phone System) with relatively lower frequency in the millimetric wave band is important in designing the high-frequency semiconductor device. Specifically, in the range of the millimetric wave band, the wavelength is 1-10 mm in the atmosphere, and the effective wavelength is approximately from 100 microns to several millimeters considering the dielectric constant of the material constituting the package. This length corresponds to the size of the high-frequency semiconductor chip, package or high-frequency circuit, so that the three-dimensional shape thereof as well as the material characteristics such as the dielectric constant and the dielectric loss have a significant influence on the high-frequency characteristic of the high-frequency package. In view of this, the design of the three-dimensional shape in the package becomes an important factor.
FIG. 1 shows a cross section of a conventional high-frequency package as the first example. The high-frequency package includes an interconnection substrate 10, a high-frequency semiconductor chip 30 where a high-frequency circuit 32 is formed, a plurality of bumps 40, and a cap 50. On interconnection substrate 10, high-frequency semiconductor chip 30 is connected by chip-bonding. Bumps 40 of high-frequency semiconductor chip 30 include signal bumps for input, output, power supply, bias and the like as well as several ground bumps. These bumps are used only for connection of high-frequency semiconductor chip 30. Cap 50 is formed of metal, for example, in order to provide not only hermetic and mechanical sealing but sealing in terms of electromagnetic wave.
FIG. 2 shows a cross section of a conventional high-frequency package as the second example. The high-frequency package is the invention disclosed in Japanese Patent Laying-Open No. 4-79255. The difference between the invention and the conventional art shown in FIG. 1 is that an electromagnetic wave absorption block 70 is provided within a cap 50 for absorbing electromagnetic waves emitted from a gap between a high-frequency semiconductor chip 30 and an interconnection substrate 10 to prevent unnecessary interference and reflection.
FIG. 3 shows a cross section of a conventional high-frequency package as the third example. The difference between this package and the conventional art shown in FIG. 1 is that a plurality of high-frequency semiconductor chips 30a and 30b are provided inside a package formed of an interconnection substrate 10 and a cap 50 in order to provide the high-frequency semiconductor with multifunction or an improved high-frequency characteristic.
FIGS. 4A and 4B respectively illustrate a cross section of a conventional high-frequency package as the fourth example and a manufacturing procedure thereof. The high-frequency package includes an interconnection substrate 10, separate packages 52a and 52b, and a guard cap 56. As shown in FIG. 4B, high-frequency semiconductor chips 30a and 30b are respectively housed in separate packages 52a and 52b, and separate packages 52a and 52b are reversed to be placed in package insert holes 28 of interconnection substrate 10. In the high-frequency package accordingly produced, a high-frequency signal line 58 at separate package 52b and a high-frequency signal line 59 at interconnection substrate 10 are connected, and the exposed portion of high-frequency signal line 59 at interconnection substrate 10 is covered with guard cap 56 as shown in FIG. 4A.
Regarding the conventional art illustrated in FIG. 1, bumps 40 connected to the periphery of high-frequency semiconductor chip 30 are used for electrical connection of high-frequency semiconductor chip 30 only. Therefore, electromagnetic waves emitted from chip high-frequency circuit 32 inside high-frequency semiconductor chip 30 leak from the gap between high-frequency semiconductor chip 30 and interconnection substrate 10. This leakage causes reflection and interference of the electromagnetic waves on the inner surface of cap 50 formed of metal, increase of noise level, generation of ripple and the like, leading to difficulty in implementation of a high performance high-frequency package having a superior characteristic such as low noise and wideband.
Regarding the second conventional art illustrated in FIG. 2, electromagnetic wave absorption block 70 having an appropriate shape is arranged at any suitable location in order to prevent deterioration of the high-frequency characteristic caused by the electromagnetic waves leaking from the gap between high-frequency semiconductor chip 30 and interconnection substrate 10. However, design of the shape and arrangement of electromagnetic wave absorption block 70 is difficult, and a series of processes of simulation, trial manufacture, and measurement evaluation should be repeatedly carried out for optimization. Consequently, the development period increases and the design cost becomes enormous. Further, a high-frequency package housing a high-frequency semiconductor chip having various functions, various sizes, and various ways of emission of the electromagnetic waves in order to meet the needs for various types of products is required, and corresponding design should be made individually. Generally, it is desirable to proceed development and design of a high-frequency package while a high-frequency semiconductor chip is developed and designed. However, manufacturing of the high-frequency package as a trial for measurement evaluation of electromagnetic wave emission is impossible unless the high-frequency semiconductor chip is available. In addition, if the number of chips obtained from a wafer is increased to reduce the cost and accordingly the size of the chip itself is reduced, or if one chip and one package are realized by assembling different chips and different packages, the chip size and the interconnection substrate should be changed. A problem in this case is that the three dimensional shape of the package should be designed again.
Regarding the third conventional art illustrated in FIG. 3, electromagnetic waves leak from the gap between high-frequency semiconductor chip 30a or 30b and interconnection substrate 10, and the electromagnetic wave from high-frequency semiconductor chip 30a and the electromagnetic wave from high-frequency semiconductor chip 30b influence each other even if the entire interconnection substrate 10 is covered with cap 50, and thus a superior high-frequency characteristic is not achieved. For this reason, the electromagnetic wave absorption block or the like may be provided. However, the increase in the number of high-frequency semiconductor chips leads to enormous difficulty in design of the electromagnetic wave absorption block. Further, the problem above cannot be solved by the electromagnetic wave absorption block only.
In the fourth conventional art shown in FIGS. 4A and 4B, separate package 52a accommodating high-frequency semiconductor chip 30a and separate package 52b accommodating high-frequency semiconductor chip 30b are formed as separate units so that design of the high-frequency package is easy. However, a connecting point is necessary between high-frequency signal line 58 of separate package 52 and high-frequency signal line 59 of interconnection substrate 10. Consequently, compared with the high-frequency semiconductor chip which is flip-chip bonded directly to the interconnection substrate, two connecting points are added per one chip between chip high-frequency circuits 32 and substrate high-frequency circuits (not shown). Generally, unnecessary reflection is likely to occur at the connecting points, and the increase of the unnecessary reflection leads to reduction in the high-frequency characteristic. In manufacturing of the separate high-frequency packages, two connections of the flip-chip bonding and the separate cap sealing are required. Additionally, one connection for mounting the individual high-frequency package onto the interconnection substrate is required. Accordingly, total three connections per one chip are required, resulting in reduction in the high-frequency characteristic. As the number of mounted high-frequency semiconductor chips increases, the number of manufacturing processes also increases, and the productivity is accordingly decreased. Further, the increase in the number of mounted high-frequency semiconductor chips leads to a relatively large size of the interconnection substrate. In terms of the high-frequency, it is desirable that high-frequency signal line 58 of separate packages 52a and 52b and high-frequency signal line 59 of interconnection substrate 10 are located on the same plane. Therefore, package insert hole 28 of interconnection substrate 10 becomes necessary and thus the number of manufacturing processes further increases.
In the conventional art illustrated in FIGS. 4A and 4B, guard cap 56 is provided at the portion of high-frequency signal line 59 on interconnection substrate 10 that is not covered with the separate packages. Since there is approximately one portion per one chip of the high-frequency signal line that is not covered with the separate packages, corresponding number of guard caps 56 should be provided.